RixStix:
We've been using YourMailingListProvider.com for 3 years without a problem.
12,000 emails per month is in the $35 range per month.
That's not too bad. MailChimp would cost me $75 a month. For that much, I might as well just keep my $105 a month 1shoppingcart account. The only thing really great about 1shoppingcart is their bulk e-mail features.
I was looking at G-Lock which is a $350 software package. But you run it on your desktop. I had a ISP send me hate mail once after using a program like this. Plus if you want serious tracking, it's an extra $35 a month. What looks appealing about G-Lock is that it say you can set it up to pull e-mails off of a mysql database. So in theory, I could maintain my client list on Zen Cart and then have G-Lock access the list on my server. I'm sure it's easier said than done though.